Mobile phone backup when landline fails

Last reviewed: 2026-03-04

Why mobile matters as a backup

Once your landline goes through a broadband router, it needs mains power to work. In a power cut, a mobile phone with battery charge is the most practical way to call for help. That makes mobile signal strength something worth checking before you need it.

Check your mobile signal at home

Try making a call from each room. Signal strength can vary within a building. Pay attention to:

• The number of bars on your phone (though this is only a rough guide)
• Whether calls drop or break up
• Whether texts send reliably

If you have a smartphone, you can check your provider's online coverage map. These are not always accurate for individual buildings, so a real call is the better test.

Provider coverage checkers

Each network operator publishes an online coverage map. Enter your postcode to see predicted indoor and outdoor signal for voice and data:

• EE (part of BT Group): search "EE coverage checker" at ee.co.uk
• Three: three.co.uk/coverage
• Vodafone: vodafone.co.uk/network/status-checker
• O2 (Virgin Media): o2.co.uk/coveragechecker

These maps show predicted signal. Actual indoor coverage depends on wall thickness, building materials, and your floor level. A basement flat will get worse signal than a top-floor room with a window.

What to do if signal is poor

Wi-Fi calling

Most modern smartphones can make calls over your Wi-Fi connection instead of the mobile network. This is called Wi-Fi calling (sometimes VoWiFi). It uses your broadband, so it works wherever your Wi-Fi reaches, even if mobile signal is zero.

The catch: Wi-Fi calling depends on your broadband router, which needs mains power. So in a power cut, Wi-Fi calling goes down along with your landline. It is useful day-to-day but not a power-cut backup.

To check if Wi-Fi calling is switched on: go to your phone's settings and search for "Wi-Fi calling". Most networks support it on phones bought in the last few years.

Femtocells and signal boosters

A femtocell (sometimes called a signal box or Sure Signal) is a small device that connects to your broadband and creates a mobile signal bubble inside your home. Some providers offer them free to customers with poor indoor signal.

Like Wi-Fi calling, femtocells depend on your broadband and power supply. They do not help in a power cut.

If nothing works indoors

If you have no usable mobile signal inside your home and no landline during a power cut:

• Go outside. Signal is usually stronger outdoors. Walk to a window, garden, or higher ground.
• Try a different network. If a neighbour has a phone on a different network, ask to borrow it. The four UK networks use different frequencies, so one may work where another does not.
• 999 uses any available network. A 999 call will connect through any mobile network your phone can reach, not just your own. Even a phone with no SIM card can call 999.
• Agree a plan with a neighbour. If you are elderly or live alone, arrange with a nearby neighbour to check on you during extended power cuts.

Keeping your mobile charged

A mobile phone is only useful if it has charge. In a power cut, you cannot plug it in.

• Power bank. A portable battery pack (power bank) can fully charge a phone one to three times depending on its capacity. Keep it charged and somewhere you can find it in the dark. A 10,000mAh bank costs under £15 and fits in a pocket.
• Car charger. If you have a car, you can charge your phone from the car's USB port or cigarette lighter socket.
• Charge before a storm. If severe weather is forecast, charge your phone and power bank to full before the power goes.

Emergency calls: what you need to know

You can call 999 from any mobile phone, even with no credit, no SIM card, and on any network. The call will route through whichever network has the strongest signal at your location.

In most power cuts, mobile masts continue working. They have backup batteries, typically lasting several hours. In a prolonged or widespread outage (lasting more than about four to eight hours), some masts may go down. This is rare but not impossible.

If you cannot reach 999 by mobile, text 999 instead. You need to register for the emergencySMS service first. Text "register" to 999 and follow the instructions. This works on all UK networks and is designed for people who are deaf or hard of hearing, but anyone can use it.

If you use a telecare alarm or other device that dials out over the phone line, mobile fallback alone may not be enough. The device itself needs a working connection to its monitoring centre. Use the device risk checker to find out which of your devices are affected.